The present invention concerns the processing of semiconductor devices and pertains particularly to the incorporation of nitrogen-based gas in polysilicon gate re-oxidation to improve hot carrier performance.
The gate of a metal-oxide-semiconductor (MOS) transistor that is formed using nitridized gate oxide has many benefits. The nitridized gate oxide exhibits superior hot carrier performance as compared to the standard silicon dioxide gate.
Nitridized oxide is formed by incorporating a nitrogen-based gas at the end of an oxidation cycle. The nitrogen-based gas is introduced at the end of the cycle because the growth of nitridized oxide is self-limiting. Once the nitrogen-based gas is introduced, approximately four to five Angstroms (.ANG.) of additional oxide can be grown. After that, no further oxidation is observed under the nitridizing environment. However, reverting to standard oxidation conditions (removing nitrogen-based gas) will allow oxide growth to proceed again.
During the fabrication of a complementary metal-oxide-semiconductor (CMOS) transistor, a slight oxidation can be performed after the polysilicon gate is patterned and etched. For sub-micron technologies, the thickness of this "poly re-oxidation" is on the order of five to ten nanometers (nm). The purpose of the poly re-oxidation is to heal any damage that may have occurred at the gate edge during the plasma etching of the gate. It also serves to act as a passivating layer over silicon and a screen for subsequent dopant implantations.
When polysilicon re-oxidation is performed, additional oxide is grown at the nitridized oxide-silicon interface in areas not covered by the polysilicon gate. Some of this oxide growth will also encroach under the polysilicon gate. In the worst case, there may be no nitridized oxide in the areas not covered by the polysilicon gate because of the combination of polysilicon over etch, along with the enhanced etch rate of the oxide during pre-polysilicon re-oxidation clean due to etch damage. This means that in the immediate vicinity under the gate edge, the oxide at the oxide-silicon interface is now just standard oxide. It is the oxide properties at this gate edge, along with the area that will eventually be under the spacer of the transistor, that will determine the hot carrier performance. Thus the polysilicon re-oxidation can compromise the advantages of using nitridized gate oxide. This is especially true when the nitridized oxide at the edges of the gate has essentially been replaced by standard oxide because the edges of the gates are crucial to hot carrier performance.